1. Field of the Invention
This invention relates to a dry etching method used in a process for manufacturing semiconductor integrated circuits, more particularly to a dry etching method utilizing electron cyclotron resonance excited by microwaves for forming a relief pattern or a recessed pattern in an etching film.
2. Description of the Related Art
In forming semiconductor integrated circuits on silicon substrates, the pattern feature size is desirably reduced in order to increase speed performance and lower production cost. The need for finer pattern features is particularly great in connection with MOS transistor gates and contact hole wiring and etching or bipolar transistor contact hole wiring and etching. The development of an etching method enabling stable formation of fine patterns with high precision is therefore been desired.
FIG. 1 shows an example of the configuration of an ECR etching machine using electron cyclotron resonance (ECR) excited by microwaves.
As shown in this figure, this type of ECR etching machine has a sample stage 35 disposed inside a treatment chamber 30 sealed by an etching chamber 31 and a discharge tube 33. The discharge tube 33 is made of quartz material. The sample stage 35 also serves as a lower electrode connected to a high-frequency power source 29. The object to be etched is placed on the sample stage 35.
The etching chamber 31 is connected with an exhauster 39 and an etching gas source 37. During etching, the treatment chamber 30 is exhausted by the exhauster 39, etching gas is then supplied into the treatment chamber 30 from the etching gas source 37, and the exhauster 39 regulates the pressure (the etching pressure) in the treatment chamber 30 to a constant value.
After the etching pressure of the treatment chamber 30 has been regulated to a constant value, a magnetron 41 is operated to generate microwaves at a frequency of, for example, 2.45 GHz and the microwaves are guided to the discharge tube 33 by a waveguide 43.
A pair of vertically separated solenoid coils (upper coil 45 and lower coil 47) is provided around the discharge tube 33. The upper coil 45 and the lower coil 47 produce a magnetic field inside the discharge tube 33 (in the treatment chamber 30).
It is known that when the frequency of the microwave guided to the discharge tube 33 is 2.45 GHz, electron cyclotron resonance (ECR) occurs at the point where the magnetic flux density of the magnetic field produced by the upper and lower coils 45 and 47 is 875 Gauss (G). This point is generally known as the "ECR point."
When plasma is formed in the treatment chamber 30 after the occurrence of electron cyclotron resonance and a high-frequency bias power (RF bias power) is applied to the lower electrode (sample stage 35) by the high-frequency power source 29, ions bombard the etching film of the semiconductor substrate thereby etching it.
Since this etching uses high-density plasma produced by electron cyclotron resonance, the ratio of isotropic etching by radicals without directivity is low while the ratio of anisotropic etching, i.e., etching by ions with directivity striking the etching film, is high.
The etching pressure is therefore lowered to promote disassociation of the etching gas and thereby raise the ion density and lengthen the ion mean free path. This is aimed at enabling the etching film to be etched without side etching or undercutting.
In the aforementioned ECR etching machine, the ECR point can be regulated by controlling the currents passed through the upper coil 45 and the lower coil 47. The intensity distribution of the magnetic field (called the "magnetic field intensity distribution" hereinafter) from the ECR point to the semiconductor substrate can also be regulated by controlling the current passed through the upper coil 45 and the lower coil 47. The range over which the ions generated in the treatment chamber 30 can move without colliding with other particles (the mean free path) can be regulated by using the exhauster 39 to control the etching pressure. The energy of the ions generated in the treatment chamber 30 and the number of ions striking the etching film can be regulated by controlling the RF bias power applied to the lower electrode (the sample stage 35) by the high-frequency power source 29. The directivity of the ions in the treatment chamber 30 can be regulated by controlling either the RF bias power or the magnetic field intensity distribution.
In the conventional dry etching method using this type of ECR etching machine, once the ECR point, magnetic field intensity distribution, etching pressure and the output of the magnetron 41 (microwave intensity) have been appropriately set for the object to be etched, they are maintained constant until the series of etching steps have been completed. In other words, these control factors are not changed in the course of the etching process.
The exception is the RF bias power, which has sometimes been varied during etching.
Conventional dry etching methods of this type are used for two types of semiconductor substrate patterning: relief patterning and recessed patterning. Relief patterning is a patterning mode in which, as in the case of forming a MOS transistor gate, the peripheral region of a circuit element portion is etched and the circuit element portion itself is not. Recessed patterning is a patterning mode in which, as in the case of forming a MOS transistor contact hole, the circuit element portion is etched to remove it from the surrounding material.
FIGS. 6 and 7 are sectional views showing the etched state of a relief pattern formed by a conventional dry etching method of this type, specifically the etched state of gates of a MOS transistor formed on a semiconductor substrate. FIG. 6 shows the etched state of gates formed at the middle of the semiconductor substrate and FIG. 7 shows the etched state of gates formed near the edge of the semiconductor substrate.
The procedures used to form the gates shown in these figures is as follows. A gate insulating film 13 consisting of a silicon dioxide film is formed on a semiconductor substrate 11. A gate material 15 constituted as a polycrystalline silicon film is formed as an etching film on the gate insulating film 13. A film of photosensitive resin (photoresist) 17 is formed on the upper surface of the gate material 15, whereafter the photosensitive resin 17 is patterned by exposure and development. The gate material 15 is etched by the dry etching method using the aforesaid ECR etching machine with the patterned photosensitive resin 17 used as an etching mask. The gate material 15 remaining on the semiconductor substrate 11 after etching constitutes the gates.
FIGS. 8 and 9 are sectional views showing the etched state of a recessed pattern formed by a conventional dry etching method of this type, specifically the etched state of contact holes of a MOS transistor formed on a semiconductor substrate. FIG. 8 shows the etched state of contact holes formed in the middle of the semiconductor substrate and FIG. 9 shows the etched state of contact holes formed in near the edge of the semiconductor substrate.
The procedure used to form the contact holes 20 shown in these figures is as follows. An interlayer dielectric thin film 23 consisting of a silicon oxide film containing phosphorus and boron is formed on a semiconductor substrate 11 as an etching film. A film of photosensitive resin 17 is formed on the upper surface of the interlayer dielectric thin film 23, whereafter the photosensitive resin 17 is patterned by exposure and development. The contact holes 20 are etched in the interlayer dielectric thin film 23 by the dry etching method using the aforesaid ECR etching machine with the patterned photosensitive resin 17 used as an etching mask. The contact holes 20 are thus formed.
In the conventional dry etching method using the aforesaid ECR etching machine, however, the etching of the etching film consisting of the gate material 15 or the interlayer dielectric thin film 23 may result in different etched sectional shapes between the middle of the semiconductor substrate 11 (FIGS. 6 and 8) and the peripheral portions (FIGS. 7 and 9).
In the etching method employing electron cyclotron resonance, the ions which effect the etching are restricted in direction of movement by and travel along the magnetic lines of force in the magnetic field. In the conventional etching method, the etching uniformity has therefore been enhanced by positioning the ECR point at a distance from the semiconductor substrate 11 and broadening the magnetic filed, thereby dispersing the plasma and preventing nonuniformity of the plasma at the ECR point from affecting the etching.
As a result, the magnetic lines of force are perpendicular to the semiconductor substrate 11 surface at the middle of the semiconductor substrate 11 but intersect obliquely with the surface of the semiconductor substrate 11 at the peripheral regions of the semiconductor substrate 11. The etching is effected by ions that travel along these magnetic lines of force and impinge on the etching film.
Since the magnetic lines of force intersect the surface of the semiconductor substrate 11 in both perpendicular and oblique directions, the etching is perpendicular at the middle region of the semiconductor substrate 11 but is skewed in the peripheral regions.
Moreover, in the conventional etching method using electron cyclotron resonance, the RF bias power produced by the high-frequency power source 29 is normally set high in order to promote etching of the etching film. As a result, the gate insulating film 13 and the semiconductor substrate 11 which are desirably left as they are without etching are also apt to undergo deep local etching.